The invention relates to an optical amplifier and an optical amplifying method used in an optical communication system or a long-distance optical transmission system and also relates to an optical transmission system using the optical amplifier.
In response to a demand for reduction of the cost of an optical communication system, a wavelength-division-multiplexed (WDM) transmission is studied. The wavelength-division-multiplexed transmission system is used for transmitting two or more signals with wavelengths different from each other through a single optical transmission optical fiber by using a multiplexing technique. An optical amplifier appropriate for the wavelength-division-multiplexed transmission system must therefore be capable of amplifying a signal to give a high S/N ratio. However, an erbium-doped optical fiber composing an optical amplifier or a semiconductor optical amplifier has a gain which is dependent upon the wavelength, giving rise to deviations in optical power among different wavelengths observed in the optical output or the gain thereof after the amplification. The deviation in optical power among signal lights with different wavelengths is particularly accumulated in a multi-stage relaying process carried out by optical amplifiers. Thus, a big deviation in optical power among different wavelengths is observed in the optical powers of signal lights after the multi-stage relaying process. As a result, the multiplexed signal light having a wavelength with the smallest optical power has a poor S/N ratio which in turn limits the maximum relayed transmission distance of the system as a whole. It is thus important to provide an optical amplifier that has no deviation in optical power among signal lights with different wavelengths.
As a conventional system with no deviation in optical power among signal lights with different wavelengths, there has been studied a system shown in FIG. 1 of a technical report of the IEICE (the Institute of Electronics, Information and Communication Engineers) OCS94-66, OPE94-88 (1994-11) with a title "Er Doped Optical fiber Amplifier for WDM Transmission Using Optical fiber Gain Control." Reference numeral 33 shown in the figure is an erbium-doper optical fiber whereas reference numerals 34 and 35 each denote an optical-optical fiber isolator. Reference numerals 36 and 37 are an optical synthesizer and an exciting optical source respectively. Reference numeral 38 denotes an optical attenuator. Reference numeral 39 is an optical coupler for splitting the output of the optical attenuator 38 and reference numeral 40 is an optical detector for detecting a split signal output by the optical coupler 39. The erbium-doped optical fiber 33, the optical-optical fiber isolators 34 and 35, the optical synthesizer 36, the exciting optical source 37, the optical attenuator 38, the optical coupler 39 and the optical detector form the conventional optical system. In this configuration, the optical fiber gain is controlled to a fixed value of 12 dB (with a power deviation among input signal lights with wavelengths of 1,548 nm, 1,551 nm, 1,554 nm and 1,557 nm set at 0 dB) by using an automatic optical-fiber-gain controller (AFGC) in order to minimize the deviation in optical power among signal lights with different wavelengths. In addition, the optical attenuator 38 is used as an automatic power controller (APC) to adjust the optical loss while maintaining the optical fiber gain at the fixed value of 12 dB so that the optical fiber-gain spectrum does not change even if the relay-amplification degree is varied.
In a real system, it is not necessarily possible to make the relay-transmission distance constant due to, among other things, conditions at the locations of relay stations. In the wavelength-division-multiplexed transmission, non-linear optical effects occurring in the course of transmission result in optical loss which is different from wavelength to wavelength and it is thus quite within the bounds of possibility that the deviation in input power is observed at the input of an optical amplifier due to a long transmission distance traveled by the signal lights.
In the conventional system, multiplexed signal lights are amplified uniformly by an exciting light source. Thus, if a deviation in input power among signal lights having the four wavelengths of the signal light is generated, the deviation in output level among signal lights having the wavelengths can not be corrected. As described above, the deviation in optical power among signal lights with different wavelengths is particularly accumulated in a multi-stage relaying process carried out by optical amplifiers. Thus, a big deviation in optical power among signal lights with different wavelengths is observed in the optical powers of the signal lights after the multi-stage relaying process. As a result, the multiplexed signal light having a wavelength with the smallest optical power has a poor S/N ratio which in turn limits the maximum relayed transmission distance of the system as a whole. That is to say, the relay-transmission distance must be shortened.
In addition, when a signal light of the multiplexed signal having one or the four wavelengths has fluctuations in power, it is impossible to suppress the fluctuations of only the signal light of a wavelength with a fluctuating power. Further, even though the automatic power controller (APC) or the automatic optical fiber gain controller (AFGC) is used in the conventional system to apply fixed control to all the multiplexed signal lights having the four wavelengths, it is quite within the bounds of possibility that fluctuations occurring in a signal light having one of the four wavelengths are dispersed to the signal lights having the other wave-lengths.
In an actual transmission system, on the other hand, in order to improve the reliability of the system as a whole or to increase the transmission capacity, in general, it is necessary to provide a spare transmission system or to build parallel transmission systems.
With the conventional technology, only a single transmission system is taken into consideration. If n parallel transmission systems are to be built, the cost of the required optical amplifiers is also multiplied n times. As a result, the total cost of the actual system is increased.